Upstream Bootstrap 3 converter

Keeping bootstrap-sass in sync with upsteam changes from Bootstrap is an error prone and time consuming manual process. This pull request is specifically concerned with automating that process as much as possible to allow a much faster release cycle.

Upstream changes to the Twitter Bootstrap project can now be pulled in using the `convert` rake task.

Here's an example run that would pull down the `3.0.0-wip` branch from the official twitter/bootstrap repo:

    % bundle exec rake 'convert[3.0.0-wip]'

The latest converter script is located [here](https://github.com/intridea/bootstrap-sass/blob/3.0.0-wip/tasks/converter.rb) and does the following:

* Converts upsteam bootstrap Less files to its matching Scss file.
* Namespaces mixin names with `twbs-` prefix to avoid conflicts with other libs.
* Copies all upstream javascripts into `vendor/assets/javascripts/bootstrap`
* Generates a javascript manifest at `vendor/assets/javascripts/bootstrap.js`
* Copies all upstream font files into `vendor/assets/fonts`

This less to scss conversion is pretty good, but not perfect. So manual fixes to the resulting Scss will necessary for now. The current version has only been tested with the official twitter/bootstrap `3.0.0-wip` branch.

See [these github issues](https://github.com/intridea/bootstrap-sass/issues?labels=converter&state=open) tagged with `conversion` which track the current shortcomings of the conversion process.